The invention relates to an automatic soldering apparatus, and, more particularly, relates to a device of the apparatus for preventing the outside air from flowing into the heating chamber of the soldering apparatus by counter flow of a small amount of an inert gas within the apparatus toward the outside air which may be abruptly caused to flow into the apparatus by change of outside conditions, thereby to maintain a constant density of the inert gas within the soldering apparatus. Thus the invention is intended to provide an extremely highly reliable automatic soldering apparatus.
Automatic soldering apparatus has been used to solder printed circuit boards with a molten solder at high temperature in air containing much oxygen. Such soldering apparatus suffers from defects and disadvantages in that molten solder and the soldered parts of printed circuit boards become oxidized and inhibit effective soldering of the printed circuit boards because of exposure of the highly heated solder and printed circuit boards to the oxygen in the air. It is therefore necessary to take subsequent laborious steps to remove the residues of solder from the soldered printed circuit boards.
Recently, the electronic parts to be mounted on printed circuit boards have been considerably smaller, and accordingly, the leads of the parts have been thinner. Therefore, even the slightest oxidization of soldered parts will cause reduced reliability of the electric products incorporating the soldered printed circuit boards.
In order to eliminate such defects and disadvantages, an automatic soldering apparatus has been provided in which an inert gas is filled to exclude oxygen from the inner part of the soldering apparatus and prevent oxidation of the highly heated molten solder and soldered parts of the printed circuit boards during the soldering operation.
In this case, the nitrogen gas used as the inert gas is expensive, and therefore the soldering apparatus is so designed as to enhance the air-tightness of the gas containment to reduce leakage of the inert gas.
However, the automatic soldering apparatus must be provided with an entrance or opening for receiving the printed circuit board to be soldered and an exit or opening for discharging the soldered printed circuit board. It has therefore been impossible to prevent the inert gas from leaking out through the entrance and the exit. Actually, it is admitted that a substantial amount of inert gas will leak out, thereby increasing costs.
Automatic soldering apparatus is controlled such that the inside temperature is progressively increased from a preheating chamber toward a reflow soldering chamber. For example, it is common for an automatic reflow soldering apparatus to have chambers of higher temperatures where the gas density is low and chambers of lower temperatures where the gas density is higher, and that the inert gas will flow toward the place of the low gas density from the place of the higher gas density.
In this case, the inert gas will flow out of the exit of the apparatus due to the difference of temperatures. This is a wasteful consumption of the expensive inert gas.
Further in this case, the flow of inert gas through the exit of the soldering apparatus will cause outside air to flow into the soldering apparatus at the entrance thereof resulting in reduction of the inert gas density in the preheating chamber. This will lower the oxidization preventing effect.
Further, automatic reflow soldering apparatus typically has a fan provided in each of the preheating and reflow soldering chambers for circulating the heated inert gas therein to melt the solder. Actually, the heated inert gas is circulated and is blown against the upper surface of the printed circuit board to preheat and additionally heat the printed circuit board so as to solder the same. The heated inert gas blown against the upper surface of the printed circuit board will separately flow in the opposite directions, that is, in the front and rear directions as the printed circuit board is transported. Such flows of inert gas will additionally increase the leakage of inert gas from the soldering apparatus.
In order to prevent the outside air from flowing into the soldering apparatus at the entrance thereof, it has been proposed to supply a much greater amount of inert gas than the amount of inert gas flowing out of the soldering apparatus at the exit thereof for the purpose of increasing a pressure within the soldering apparatus.
However such a method requires an extremely large amount of inert gas and is uneconomical.
Even in the above mentioned method, it is required to limit the supply of the inert gas to a degree such that the inert gas flowing out through the entrance of the soldering apparatus will be kept to a minimum. This method is not able to account for a sudden change of the environment outside of the apparatus. For example, if a wind blows, outside air will flow into the soldering apparatus at the entrance thereof.
The present invention has been provided to eliminate the defects and disadvantages of the prior art. It is therefore a primary object of the invention to provide an inert gas jetting nozzle or nozzles at each opening, that is, in the printed circuit board receiving station and in the printed circuit board discharge station of the automatic soldering apparatus, the nozzle or nozzles being directed to the entrance and the exit respectively. The inert gas jetted from the nozzle or nozzles will prevent the outside air from flowing into the soldering apparatus at the entrance and the exit thereof and thereby maintain the inert gas density at a desired constant level in the gas chambers of the soldering apparatus. Thus the invention is intended to prevent the oxidization of the molten solder and of the soldered parts of the printed circuit board, and at least to eliminate the diverse of soldering effects.
It is another object of the invention to provide a sensor in each of the entrance and the exit for detecting the velocity and direction of outside air flowing through the entrance and the exit to cause the inert gas to be jetted from each of the nozzles, the consumption of the jetted inert gas being limited to a minimum needed to shut off the outside air from the entrance and the exit. Thus the invention is intended to instantly cope with an abrupt change of the outside environments.